AN4105 Ver la hoja de datos (PDF) - Fairchild Semiconductor
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AN4105
Fairchild Semiconductor
AN4105 Datasheet PDF : 22 Pages
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AN4105
APPLICATION NOTE
4.2 Discontinuous Conduction Mode (DCM)
The appearance of an interval in which the inductor current
becomes zero during a switching cycle marks flyback
converter operation as discontinuous conduction mode
(DCM). As shown in Figure 17, the voltage waveform
applied to the inductor, VLm, becomes more complex in
DCM. Hence, to avoid difficulties in computation TOFF is
not used. Instead, three input and output relationships of a
converter are derived by using TOFF * , the time when the
output rectifier diode is actually conducting.
VGS
0
ON
OFF
VDS
Vi+nVO
Vi
0
VLm
0
A
Vi
B
nV0
Vi+nVO
VD
0
VO+Vi/n
TON
ILm
SSlloppoee=Vi/Lm
0
IDS
* TOFF
VO
TOFF
Slope=nVO/L m
0
ID
IC
0
IO
0
t0
t1
t2
t3
Figure 17. Flyback converter operating waveforms in
discontinuous current mode (DCM).
The boundary condition between DCM and CCM is:
Ii + IO
=
---V-----i--
2Lm
TO
N
The following input output relationship in DCM is derived
by using the fact that the colored areas A and B of VLm in
Figure 17 must always be equal because, in steady state, the
average inductor (or transformer) voltage is always zero.
.
ViTON = nVOT*OFF
V-----O--- = ----T----O----N------ = -----D-----*------
Vi T*OFF 1 – D*
Deriving the above equation again, using Io and the fact that
the input and output powers are equal, Vo is obtained as:
VO
=
---------------------(--V-----i--T----O---N----)---2---------------------
2
I---O--
n
Lm(
TON
+
TOFF)
+
Vi
The following equation represents the input power:
PIN
=
1--
2
Lm
IL2 m,(peak)fsw
where fsw is the switching frequency.
4.3 Flyback Converter Design
4.3.1 Turns Ratio Considerations
The turns ratio of an SMPS’s flyback converter transformer
is an important variable. It affects the voltage and current
levels associated with the primary side switching device and
the secondary side rectifier, as well as the number of turns on
the transformer and the current through it. A frequently
discussed design concept suggests operating at maximum
duty ratio when the input voltage is a minimum. For
simplified calculations, here it is assumed that operating
conditions change as listed immediately below.
- Vac input: 85 ~ 265Vac
- Vdc (rectified voltage): 100 ~ 400Vdc
- Output voltage: 50Vdc
- Inductor current: Continuous conduction mode (CCM)
operation assumed.
The input power taken by the dc source is the product of the
dc voltage and average input current. Using a wide duty
cycle to deliver equal average current reduces efficiency.
A narrow duty cycle increases the effective current on the
primary side, increasing the operating temperature of the
primary winding and the MOSFET. Also, it is best to decide
on a turns ratio, n, based on the device used. If the voltage on
the primary side MOSFET is relatively low (e.g., 600V),
make n small; if it is on the high side (e.g., 800V), make n
large. As the value of n increases, the primary side switching
device current and the secondary side rectifier diode voltage
decreases Hence, with high output voltage and multiple
secondary side outputs, it is advantageous to increase n.
14
©2002 Fairchild Semiconductor Corporation
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